A switched reluctance machine (SRM) is a brushless, synchronous machine having salient rotor and stator poles. There is a concentrated winding on each of the stator poles, but no windings or permanent magnets on the rotor. Each pair of diametrically opposite stator pole windings is connected in series or in parallel to form an independent machine phase winding of the multiphase SRM. Ideally, the flux entering the rotor from one stator pole balances the flux leaving the rotor from the diametrically opposite stator pole, so that there is no mutual magnetic coupling among the phases.
Torque is produced by switching current in each phase winding in a predetermined sequence that is synchronized with angular position of the rotor. In this way, a magnetic force of attraction results between the rotor poles and stator poles that are approaching each other. The current is switched off in each phase before the rotor poles nearest the stator poles of that phase rotate past the aligned position; otherwise, the magnetic force of attraction would produce a negative or braking torque. Hence, by properly positioning the firing pulses relative to rotor angle, forward or reverse operation and motoring or generating operation can be obtained. Typically, the desired phase current commutation is achieved by feeding back the rotor position signal to a controller from a shaft angle transducer, e.g. an encoder or a resolver. To improve reliability and to reduce size, weight, inertia, and cost in such drives, it is desirable to eliminate this shaft position sensor. To this end, various approaches have been previously proposed for indirect rotor position sensing by monitoring terminal voltages and currents of the motor. One such approach, referred to as waveform detection, depends upon back electromotive forces (emf) and is, therefore, unreliable at low speeds and inoperative at zero speed.
Another approach to indirect rotor position sensing is disclosed in commonly assigned U.S. Pat. No. 4,772,839, issued Sep. 20, 1988 to S. R. MacMinn and P. B. Roemer, which patent is incorporated by reference herein. The cited patent describes an indirect position estimator for a SRM which applies low-level sensing pulses of short duration to the unenergized motor phases. Application of the sensing pulses results in a change in current in each of the unenergized phases. The change in current is sensed by a current sensor and an estimated inductance value is derived therefrom. A pair of estimated rotor angles corresponding to the estimated inductance value for each of the unenergized phases is ascertained. One such pair is shifted by a value equal to a known phase displacement of the other unenergized phase. The pairs of estimated angles are then compared to determine which of the angles match. An estimated instantaneous rotor angular position equal to the matching angle is produced. Moreover, in case any of the stator phases undergoes a change in state during sampling or in case two phases do not remain energized throughout the sampling, an extrapolator is provided to generate an extrapolated rotor angular position instead of the estimated position.
Still another approach to indirect rotor position sensing is disclosed in commonly assigned U.S. Pat. No. 4,959,596, issued to S. R. MacMinn, C. M. Stephens and P. M. Szczesny on Sep. 25, 1990, which patent is incorporated by reference herein. According to U.S. Pat. No. 4,959,596, a method of indirect rotor position sensing involves applying voltage sensing pulses to one unenergized phase. The result is a change in phase current which is proportional to the instantaneous value of the phase inductance. Proper commutation time is determined by comparing the change in phase current to a threshold current, thereby synchronizing phase excitation to rotor position. Phase excitation can be advanced or retarded by decreasing or increasing the threshold, respectively.
Even more recent approaches to indirect position estimation have been described in U.S. patent application Ser. No. 653,374 of J. P. Lyons and S. R. MacMinn, now allowed, and U.S. patent application Ser. No. 653,371 of J. P. Lyons, M. A. Preston and S. R. MacMinn, now allowed, both filed Feb. 11, 1991 and assigned to the instant assignee. The indirect position estimating methods of the hereinabove cited Lyons et al. patent applications, which are incorporated by reference herein, each avoid active probing of the motor phases since such active probing usually imposes speed limitations on the machine. For example, according to Lyons et al. patent application Ser. No. 653,374, instantaneous phase current and flux measurements are performed in a predetermined sequence that depends on the particular quadrant of operation, i.e. forward motoring, reverse motoring, forward generating, or reverse generating. For each phase in the predetermined sequence of sensing, phase flux and phase current measurements are made during operation in a pair of predetermined sensing regions, each defined over a range of rotor angles. Rotor angle estimates are derived from the phase flux and phase current measurements for each respective phase during the respective sensing regions thereof. The rotor angle estimates for each phase are normalized with respect to a common reference phase, and a rotor position estimate for the SRM is computed therefrom.
Alternatively, the method of Lyons et al. patent application Ser. No. 653,371 involves a flux/current model of the machine, which model includes multi-phase saturation, leakage, and mutual coupling effects. The flux/current model includes a network mesh of stator, rotor and air gap reluctance terms. The network is driven by magnetomotive force terms corresponding to the ampere-turns applied to each of the stator poles. Phase current and flux sensing for each phase are performed simultaneously. The reluctance terms of the flux/current model are determined from the phase flux and current measurements. The phase current and flux measurements also determine the rotor position angle relative to alignment for each respective motor phase and which phase (or phases) is operating in its predetermined optimal sensing region defined over a range of rotor angles. The measurements on at least two phases are then used for establishing whether the stator phases of the sensing phase are approaching alignment or maximum unalignment with SRM rotor poles. Finally, the rotor position angle for the sensing phase and its position relative to alignment are used to provide a rotor position estimate for the motor.
The hereinabove described position estimation methods of the Lyons et al. patent applications may be conveniently implemented using a microprocessor. However, upon initialization, the microprocessor must take a series of measurements before the position estimates are sufficiently reliable. Such an initial acquisition sequence results in a period of time for which valid position estimates are not available. Furthermore, operation of a microprocessor can result in a variety of so-called soft-errors which can cause erratic operation; normal operation in the event of such soft-errors is resumed by resetting and restarting the microprocessor. It is desirable, therefore, to provide means for monitoring the position estimates produced by such a microprocessor-based estimator and detecting a loss of lock condition wherein the estimator is no longer accurately tracking rotor position.